Some time back while browsing for something completely different I found this application called FreeMind. As it turns out I converted most of my text files into FreeMind mind maps and haven't turned back since =).
I love the way you can move nodes to change the tree structure and hide nodes you don't want to see. Or the feature where you can link to other FreeMind documents, web addresses and folders and have them open as you would expect them to.
Another neat thing is if you want to export the data to plain tab-structured text, you just copy-paste from the tree.
I'd recommend anyone who has to keep notes (for projects, studies, ... anything really), to go and check out FreeMind at http://freemind.sourceforge.net.